1. Field of the Invention
The present invention relates to an electroacoustic transducer, and more particularly, to an electroacoustic transducer, in which a gap between an elastic plate constituting a diaphragm and a top of a pole piece is formed to be extremely precise, and in which the gap length will not vary due to environmental change such as temperature.
2. Description of the Related Art
A structure of an electroacoustic transducer as an example of the prior art is illustrated in FIGS. 13 and 14. There is an upper case 201, and formed at the center of the FIG. 13 is a sound port 203. There is also a lower case 205 under the upper case 201 in FIG. 13, and the lower case 205 and the upper case 201 are welded and fixed by ultrasonic welding. There is an opening 207 formed at the bottom of the lower case 205 in FIG. 13. A base 209 and a core 211 are integrally (namely, the separate parts have been integrated in advance) secured to the opening 207 as a "pole piece" 212. A board 213 is also attached on the bottom surface of the base 209 in FIG. 13.
A coil 215 is wound around the core 211. Coil terminals 215a and 215b of the coil 215 are respectively secured, for example by means of solder welding, to lead terminals 217 and 219 attached to the board 213. A magnet 223 is placed around the coil 215 with an existence of a ring-like clearance 221 formed in between. The aforementioned lower case 205 is provided, at the inner periphery thereof, with a step portion 225 at which a diaphragm 227 is supported. The diaphragm 227 comprises an elastic plate 228, and a magnetic piece 229 which is attached as an added mass to the center portion of the elastic plate 228. The magnetic piece 229 is welded and fixed, for example by means of spot welding, to the elastic plate 228.
In the thus constituted electroacoustic transducer, the elastic plate 228 integrally provided with the magnetic piece 229, is set to have a given polarity by means of the magnet 223, and hence, is attracted to the magnet 223. When a current flows across the coil 215 via the lead terminals 217 and 219 under this situation, the core 211 is magnetized, generating a magnetic field at the distal end thereof. When the magnetic pole of the core 211 induced by the coil 215 is different from the magnetic pole induced by the magnet 223 attached to the elastic plate 228, the elastic plate 228 is attracted to the core 211. When the magnetic pole of the core 211 induced by the coil 215 is the same as the magnetic pole induced by the magnet 223 attached to the elastic plate 228, the elastic plate 228 repels the core 211.
Consequently, by allowing the current to intermittently flow in either direction, the elastic plate 228 repeats the above-discussed operation. In other words, the elastic plate 228 vibrates at a given frequency, thus generating a sound.
In regard to the electroacoustic transducer having the above discussed structure and function, each constituent part thereof is classified into a "structural section", a "magnetic circuit section", an "electromagnetic coil section" and an "acoustic circuit section" according to the function.
In detail, the structural section includes the lower case 205, the base 209, the elastic plate 228 and the core 211.
Similarly, the magnetic circuit section includes the base 209, the elastic plate 228, the core 211 and the magnet 223.
The electromagnetic coil section includes the coil 215, the lead terminals 217 and 219, and the board 213.
For reference, the identical part may be included in different sections at the same time.
With regard to the constituent parts which belong to the magnetic circuit section, the magnetic flux from the magnet 223 as well as the magnetic flux by magnetomotive force from the coil 215 are required to effectively be induced in the magnetic circuit. Thus the constituent parts included in the magnetic circuit section, namely, the base 209, the elastic plate 228, the core 211 and the magnet 223, are made of material having high magnetic permeability, like metal such as steel or nickel.
Further, as for the constituent parts which belong to the structural section, the following conditions are required. Firstly, with regard to the elastic plate 228, the efficient vibration by means of magnetic force between the magnetic circuit section and the electromagnetic coil section is required, hence the elastic plate 228 is required to have remarkable flexibility as well as rigidity. Secondly, with regard to the other constituent parts in the structural section, a gap between the elastic plate 228 and the core 211 (as shown by letter L in FIG. 13) is required to be extremely precise, as well as to prohibit variation of length of gap L due to environmental change such as temperature. Therefore the constituent parts in this section may preferably be made of high-workability engineering plastic material, such as polyphenylene oxide (PPE), nylon, liquid crystal polymer (LCP), etc.
However, as above discussed, the base 209, the elastic plate 228, and the core 211 in the structural section also belong to the magnetic circuit section, and because of functional priority to be the parts of the magnetic section, the high magnetic-permeability material such as steel or nickel (i.e. metal) is ordinarily used for these parts.
As for the lower case 205, which is in the structural section but in the magnetic circuit section, the engineering plastic material such as PPE, nylon, LCP is used.
The above discussed prior art has the following problem.
As discussed above, in the case of the electroacoustic transducer in the prior art, the pole piece 212 is made of metal, and on the other hand, the lower case 205 is made of engineering plastic material such as PPE, nylon, LCP. Accordingly, when the dimension of each part varies due to change in temperature, etc., since the thermal expansion rate of each part is different, the amount of variation of dimension (especially the amount thereof in the axial direction) becomes different from each other, thereby the length of the gap L in FIG. 13 between the elastic plate 228 and the core 211 varies. When the length of the gap L varies, the acoustic characteristic such as sound pressure or frequency may also vary, and eventually the desirable acoustic characteristic may not be obtained.